It is a reusable sticky putty like substance. Called White tac in some countries. used to stick kids drawings on refrigerators.
The stuff copes reasonably well with heat, will support the single pin socket enclosing the pin when you turn the PCB over to solder, and won’t leave an oily residue on your PCB.
I think most supermarkets sell the product.
Edit: As an aside, I find the stuff really useful for holding SMD chips in position until the first pins are soldered. I don’t know if there are long term corrosive effects if you place the stuff on IC pins, but I avoid that anyway.
ok, it’s very interesting.
Salvatore
Just a word of advice -
Buy a “Minus set” of pin gauges. Like this:
https://www.amazon.com/Accusize-Tools-0-011-0-060-Steel/dp/B00JOLCSF6/ref=sr_1_1_sspa?crid=2ULOI6G2UQNG6&keywords=minus%2Bpin%2Bgauge%2Bset&qid=1695046581&sprefix=minus%2Bset%2Bpin%2Caps%2C289&sr=8-1-spons&sp_csd=d2lkZ2V0TmFtZT1zcF9hdGY&th=1
I own two sets, which cover .005” - .5”
Very handy for measuring exactly how big a PCB hole is.
currently unavailable, but diameters are too small for my needs.
Thank you
Salvatore
They are available in many size ranges.
Just search for “pin gauges”:
e.g:
https://www.amazon.com/HFS-Steel-Minus-250Pcs-0-251-0-500/dp/B0C3H5G4N6/
not useful, they are all imperial and they have a very high cost. But after all I have to measure a component to fit in a hole, not the contrary.
Thank you anyway.
Salvatore
You also do not really have to have a hole test PCB just for this.
You can just add extra holes to any PCB order to have a bunch of holes to test with.
And any holes bigger then 1.2mm or smaller then 0.9mm won’t be useful either.
And don’t forget:
If you have some small diameter drill bits, the diameter of the shank is usually the diameter of the hole you drill, so place the shanks of the drill bits into the hole 'till you find a suitable fit and measure the shank. This method works the same as the pin gauges without the cost of the pin gauges.
To be honest, we are discussing tolerances of a PCB, not an aircraft jet turbine motor. 
alas, the project where I’m using those pins has been updated to reduce the overall size to a minimum, so there is no room for extra pads.
Salvatore
ok, it is the best solution.
Salvatore
Just remember that the finished size on the PCB will be smaller- when you specify a “.5mm” hole, you are asking for “finished size.” The hole is drilled oversize to allow for plating.
yes, anyway JLC told me that if I request, for example, a 1 mm. hole, it will be 1 mm. (+0.13mm/-0.08mm. tolerance) plating included, that’s to say: finished.
Salvatore
There are many different types of Pins for PCB’s - the decision of Choice depends on a handful of considerations and are not to be taken too lightly.
I prefer Knurled-Press-Fit and use some with Threaded Hole for receiving Screws and threaded pins.
There are different types of Knurling designed for specific reasons.
You can search Mouser, Digikey and Amazon to help decide and look at their Datasheet’s…
Below shows the one’sI use so, I made footprints for them…
Different Knurling’s
That is what you get with standard PCB pooling services. That tolerance is simply too big to guarantee that there is a press fit at all.
Just to give you an idea of the difference of the tolerance for “real” press fit holes:
Press fit hole PCB, or called press fit pins PCB, they’re component holes with copper plated through and with tighter tolerance than standard PTH, the tolerance of standard PTH is +/-0.075mm(3mil), but the tolerance of press fit hole is normally +/-0.05mm(2mil).
Source:
Also, how many of these PCB’s are you making? Those Chinese factories are very cheap for small orders, but once you go up to a few hundred PCB’s, then it’s not so clear cut anymore. For those number of PCb’s, the PCB’s often get so cheap that the parts on them become the mayor cost.
very interesting. Where are the cubic pins with screw, Mouser, Amazon? Search with…?
Salvatore
ok, I see, but tight tolerances put the PCBs out of my poor budget… 
I will do the best that I can with various drill bits.
Thank you.
Salvatore
thank you for the information.
Salvatore